The coating of substrates with doped tin oxides is well known whereby the electrical resistance of the surface, so coated, is decreased and the infrared reflection is increased. Technically, these physical properties are used for heat-absorbing glazing or also for the two-dimensional heating of panes, such as windshields or rear windows of cars and refrigerated counter glazing.
In order to produce such layers, suitable tin compounds (basic compounds) are brought, preferably simultaneously with a doping agent, into contact with the glass surface, which has been heated to 400.degree. to 800.degree. C. The basic tin compound forms a coherent tin(IV) oxide layer on the surface of the glass, the glass ceramic or the enamel. Particularly fluorine, as doping agent, increases the electrical conductivity and brings about the high infrared reflection. The use of powdery dibutyl tin difluoride is described in the EP-A-0 039 256 and EP-A-0 511 044.
The coating itself can become homogeneous only if the powder flows well in the coating equipment and does not agglomerate within it. Due to hygroscopy, alkali and alkaline earth impurities, in particular, are responsible for the agglomeration. Furthermore, alkali ions interfere with the formation of the electrically conductive and, with that, infrared-reflecting layer, since they act as defect sites for the free electrons, necessary for the charge transport, and, with that, lead to a reduction in the functional properties of the layers.
Dibutyl tin difluoride (DBTF) usually is synthesized from dibutyl tin dichloride (DBTCL) and potassium fluoride in a water/alcohol solvent. Furthermore, sodium fluoride or other alkali or alkaline earth fluorides are also used instead of potassium fluoride; however, they lead to the problems mentioned above.
In the U.S. Pat. No. 4,694,091, a method is described for synthesizing DBTF from an alcoholic solution of DBTCL and an aqueous solution of ammonium difluoride. At the same time, the initially-formed DBTF is dissolved again in a suitable solvent, such as methanol, and recrystallized at least once. During the recrystallization, silica gel, preferably Aerosil.RTM. 972, is added in order to support the formation of the desired particle size. During the subsequent crystallization, a temperature jump of at least 45.degree. C. is regarded as necessary and achieved by pouring or even spraying the hot solution into a cooled, clean solvent. In a subsequent step of the process, washing is repeated once more with chlorinated hydrocarbons, such as methylene chloride or trichlorotrifluoroethane. With this effort, it is then possible to obtain a DBTF powder with a suitable average particle size between 15 and 25 .mu.m. This powder consists of spherical particles and is thus macroscopically flowable and can be metered out.
Such a method is not satisfactory from several points of view. Particularly, the expensive conduct of the process and the use of large amounts of methanol and chlorinated hydrocarbons are disturbing features.
A method for reacting dibutyl tin diacetate with hydrofluoric acid is described in U.S. Pat. No. 4,322,363.
In the EP-A-0 364 337, a synthesis method is described, for which the DBTCL is reacted with hydrofluoric acid in a suitable solvent. The further steps of the process comprise the use of o-dichlorobenzene for the purpose of driving out excess hydrofluoric acid and crystallizing the solution, which had previously been heated to 120.degree. C., by cooling. The product must subsequently be washed once again with halogenated hydrocarbons.
It is furthermore known to those skilled in the art that organic tin fluorides can also be obtained directly by the reaction of organic tin oxides with hydrofluoric acid. With respect to the starting compounds, this is the most direct way.
In practice, on the other hand, this method is unsuitable, since it does not lead to the desired powder properties of flowability and particle size and instead produces agglomerated, practically no longer flowable, pasty powders.
Since organic tin oxides are hydrophobic compounds, the reaction in aqueous media frequently is difficult, because the degree of conversion achieved is not sufficient. Frequently, therefore, solvent mixtures such as water/alcohol are used, in order to achieve a corresponding degree of conversion. Accordingly, the use of organic solvents is also necessary for this method. The necessary working up increases the costs of the process. Furthermore, the products obtained from this process tend to sinter or adhere during drying, so that the fineness of the particles, required for the application, is not achieved and such products accordingly cannot be used for coating glass.
All the methods mentioned prove to be technically complicated. In addition, expensive purification steps are required, generally involving the use of halogenated hydrocarbons or expensive starting materials. There was therefore a need for a suitable method, which avoids the disadvantages mentioned above. These problems are solved by the inventive synthesis method by which, in particular, an alkali-free and alkaline earth-free DBTF with spherical particles of suitable average particle size is achieved.